A semiconductor light emitting elements has small size, lightweight, low power consumption and long life span, and does not require preheating time and driving circuits. Further, since the semiconductor light emitting element is resistant to impact and vibration and can be packaged into various shapes, the semiconductor light emitting element is expected to substitute for a backlight of a large-sized liquid crystal display, a general illuminator and a light source for a vehicle headlight within next few years.
In particular, a nitride-based semiconductor light emitting element has excellent characteristics of electron affinity, electron mobility, electron saturation velocity and electric field breakdown voltage, so that high efficiency and high power can be implemented. Since the nitride-based semiconductor light emitting element does not contain harmful substance such as As or Hg, it has been noticed as an environment-friendly element.
However, nitride semiconductor light emitting elements that have been developed up to now are still unsatisfactory in view of high power, light emitting efficiency and price, and their performance should be more improved. Particularly, in order for the light emitting elements to substitute for conventional mercury lamps and fluorescent lamps, a problem of high power and thermal stability in accordance therewith should be solved.
In general, a nitride semiconductor light emitting element is fabricated by sequentially laminating a nitride n-type layer, a nitride active layer and a nitride p-type layer, and then disposing two electrodes horizontally so as to apply power to the n-type and p-type layers.
Although fabrication process of such a light emitting element with a horizontal structure is relatively simple and thus cost effective, but it is difficult to implement high power. That is, since light emitted from the active layer is absorbed in the two electrodes and thus is not emitted to the outside, the high power cannot be implemented. In addition, a sapphire substrate has a low thermal conductivity and thus heat generated during the operation can not be effectively emitted, which lowers the thermal stability.
In order to solve such problems, a light emitting element with vertical structures and flip-chip type light emitting elements have been suggested. In this case, a reflective layer is formed on any one electrode to allow light generated from an active layer to be easily emitted to the outside, whereby utilization efficiency of the light can be enhanced. Further, a metal substrate with excellent thermal conductivity is used in place of a sapphire substrate, so that the thermal stability can be enhanced.
Thus, in order to achieve higher power, the development of an electrode having high light reflectivity should be preceded. Since Al or Ag metal is excellent in a visible ray region in view of light reflectivity, an excellent characteristic of light output can be achieved by using such a metal as an electrode. However, since Al has large contact resistance with a nitride-based semiconductor layer, it is difficult to supply large current. On the contrary, although Ag has lower contact resistance, it has poor interlayer adhesive strength and low thermal stability. Therefore, agglomeration, interface voids and the like are formed during high temperature heat treatment.
Due to such problems, Ni/Au based electrodes used for light emitting elements having a horizontal structure is still used. Therefore, it is limited to ensure light output to the extent where the light emitting elements can substitute for conventional white light sources.